This invention relates to an electron beam exposure system which exposes a target mounted on a table such as a wafer or a photomask, using an electron beam, at a high speed and with a high accuracy in order to draw a desired circuit pattern on the target.
Recently electron beam exposure systems of various types have been developed, which minutely process a target such as a semiconductor wafer or a photomask. Of these systems, the electron beam exposure system of the hybrid raster scanning type is said to be one of the most advantageous system in respect of drawing speed and accuracy. The system of this type, however, is disadvantageous in that both the drawing speed and the drawing accuracy are limited. Particularly, the drawing speed cannot be so much increased.
In an electron beam exposure system of the conventional hybrid raster scanning type, a target mounted on a table is exposed to an electron beam as the table is moved in +Y direction at a constant speed. The beam is deflected at a predetermined angle .theta. to an X direction normal to +Y direction. The deflection angle .theta. is determined by a scanning speed at which the electron beam scan the target and by the speed at which the table is moved. The angle .theta. is such that the locus of the electron beam, which is projected on the target, extends in X direction. The angle .theta. is not varied since the beam scans the target at a constant speed and the table is moved at a constant speed. Another system of the hybrid raster scanning type is disclosed in Japanese Patent Disclosure (Kokai) No. 117685/1979 published on Sept. 12, 1979, in which the deflection angle .theta. is changed according to deviations of the speed at which the table is moved. As the table is moved in +Y direction at a speed, one frame is drawn on the target mounted on the table. Upon completion of the drawing of the frame, the table is decelerated and is then moved on step in X direction for a distance equal to the scanning pitch. Then the table is accelerated in -Y direction to reach a constant speed. During the table is moved in -Y direction at the constant speed, the next frame is exposed. This sequence of operations is repeated until the entire surface of the target is exposed to the beam and the desired circuit pattern is drawn on the target.
In the system described in the preceding paragraph the table may be moved at an increased speed in order to shorten the time to expose the target to an electron beam. If the table is moved too quickly, however, the circuit pattern will not accurately be drawn on the target. Besides, the system must be large. It will be described why these problems arise in the conventional system.
As mentioned above, the table is moved at a constant speed while the electron beam scans the target to draw one frame of a desired circuit pattern, and upon drawing the frame the table is decelerated and then accelerated. Such rapid deceleration and acceleration of the table are undesirable. They would damage the mechanism for moving the table and would render the same gradually unstable. Accordingly, both the deceleration and the acceleration must be slowed down. For moving the table at a high constant speed, the acceleration and deceleration periods must be relatively long. Thus the time required for exposing the entire target cannot be shortened even if the table is moved in +Y direction at a relatively high speed. If the deceleration and acceleration periods are relatively long, a larger space is required, in which the table is moved. If a larger space is required, a housing defining the space must be proportionally large. For this reason the system has to be large.
Furthermore, if the table is moved in +Y direction at a higher speed, a more time lag is inevitable between the detection of the table and the start of electron beam control. The longer is such time lag, the more will be deteriorated the exposure accuracy.